Electrical energy production through solar conversion is of ever greater need in light of global warming resulting from the use of fossil fuel. However, energy production from solar conversion is often more expensive than a comparable production from fossil fuels. One major factor in the balance between fossil fuel use and the cleaner and more environmentally friendly solar conversion is the cost of the necessary solar conversion modules and their efficiency. The highest efficiency solar modules use a silicon wafer substrate derived from semiconductor grade silicon. Given the need to reduce global warming and the resulting demand for solar modules, semiconductor grade silicon prices have increased markedly. Lower cost solar modules can be produced using thin-film techniques but so far such modules are neither as efficient nor as long-lived as traditional silicon-wafer-based solar modules. The lack of efficiency for thin-film solar modules is problematic during winter and cloudy conditions. Accordingly, there is a pressing need for more efficient and cost-effective semiconductor grade silicon production techniques. In this fashion, environmentally friendly solar conversion can compete with superficially less expensive fossil fuel uses.
Conventional semiconductor grade silicon production techniques are not only costly but often create environmental problems resulting from toxic waste byproducts. Production of ultrapure polysilicon commonly begins with a trihalosilane feedstock that is thermally decomposed into silicon and a tetrahalosilane byproduct. For example, in the Siemens process, trichlorosilane is reduced to produce silicon with tetrachlorosilane as the byproduct. This process was traditionally open loop and thus resulted in significant hazardous waste production. The production of polysilicon through the Siemens process in less-developed countries has lead to alarming environmental degradation with open and notorious dumping of the toxic byproducts. In addition to its polluting nature, the Siemens process also demands considerable use of energy because it is a relatively-high temperature process.
In contrast to the open-loop Siemens process, the tetrahalosilane byproducts are reused in a closed-loop process. A closed-loop process is thus inherently less polluting and uses less energy. For example, U.S. Pat. No. 4,318,942 (the '942 patent) discloses a closed-loop polysilicon production process in which tribromosilane (TBS) is thermally decomposed into silicon, silicon tetrabromide (STB) and hydrogen gas (H2). It may be observed that STB may also be denoted as tetrabromosilane but such a designation will have the same acronym (TBS) as tribromosilane and is thus avoided herein. As disclosed in the '942 patent, the tetra-halogenated byproduct STB along with the hydrogen gas byproduct are recycled in a converter using metallurgical grade silicon (MGSi, 95% pure) according to the reaction of:MGSi+3SiBr4+2H2→4SiBr3H  (1)to produce TBS, which may then be thermally decomposed as just described. In this fashion, the impure silicon in the converter transforms into ultra-pure polysilicon with no halogenated silane byproducts—these byproducts are entirely recycled such that STB becomes TBS, which is then produces STB as a byproduct during thermal decomposition, and so on. The resulting closed-loop reuse of the silicon tetrabromide byproduct is not only environmentally friendly, it is energy efficient as well in that TBS thermally degrades at lower temperatures as compared to those required for trichlorosilane techniques. In addition, TBS production of silicon advantageously produces a preferable granular form factor for the resulting silicon as compared to traditional chloro-silane approaches.
Although the closed loop process disclosed in the '942 patent is thus advantageous, some inefficiency results from the recycling of the STB and hydrogen byproducts in that these byproducts are in ultra-pure form yet they are reacted with relatively impure silicon to produce TBS. The resulting TBS must then be purified such as through distillation before it can be thermally decomposed into silicon. This purification requires a substantial amount of energy.
Accordingly, there is a need in the art for improved closed-loop polysilicon production techniques.